The invention relates to sub-macro cellular layers of a wireless communications system. In particular, the invention relates to a sub-macro cellular base station network for third generation cellular and TDD mode cellular wireless communications, for the communication of voice and data. The invention also relates to sub-macro cellular communications networks and Wireless Local Area Networks (WLANs) capable of supporting discontinuous and/or bursty data transmission.
The term xe2x80x9cwireless communications networkxe2x80x9d is used to refer to a communications network which is divided into geographical cells or areas. For example, cellular communications networks are usually divided into cells which are geographical areas each of which contains a base station transceiver (BST). Mobile and fixed subscriber stations located in the communications network communicate with one or more base station transceivers, for example, the closest base station transceiver. Each base station transceiver has a limited range and a cell can be thought of as a geographical region over which a base station transceiver is intended to operate effectively. Cellular communication network cells can be split up further in regions of poor coverage and/or high demand: sub-cells in the region of 100-400 metres radius are known as micro-cells; sub-cells in the region of 50-100 metres are known as pico-cells. For convenience the largest cell unit shall be referred to as a macro-cell smaller cells either pico or micro are hereinafter referred to as xe2x80x98sub-macroxe2x80x99 cells. FIG. 1 shows the inter-relationship of the various cells in a cellular communications system 10. Macro-cells are identified as 12, within which micro-cells 14 lie: within the micro-cells lie pico-cells 16. These can provide overlapping coverage between themselves and the overlying cell. Similarly in a WLAN, mobile and fixed subscriber stations (which will be referred to hereinafter as terminals) communicate with base stations, the number of WLAN base stations per terminal typically being greater than is the case for cellular communication networks, to cater for the smaller size of areas of coverage and to take into account the greater effect of the internal walls of buildings and the like which block or reduce signal propagation and, indeed, are present as signal reflectors.
Mobile terminals such as mobile telephones may be located within a cellular communications network and send radio signals to and receive radio signals from base station transceivers. Each mobile station operating within a cell requires a certain amount of bandwidth to operate and because the total bandwidth of base station transceivers is limited the number of mobile terminals which can operate within a cell is limited.
In general an object of wireless communications system design is to reduce the number of base station sites required by increasing their range and or capacity. The term, xe2x80x9ccapacityxe2x80x9d is used herein to refer to any suitable measure which provides an indication of how many conventional mobile terminals or other terminals are able to communicate effectively with a given antenna arrangement. Macro base station sites for cellular systems are particularly expensive, both in terms of the equipment required and the need for a geographical site for each cell site where, inter alia, large structures are frequently employed where planning permission is required. In the case of micro-cell and pico-cell applications which are frequently indoors, the installation costs can be moderate. In the case of data communications, the requirement for an instantaneous high bandwidth of, typically, short duration can cause problems. That is to say, systems are increasingly required to operate under bursty traffic conditions where the number of active users is only a fraction of the potential ones.
Various TDD modes have been proposed for the 3rd generation cellular mobile radio component of the ITUR International Telecommunications Union (Radio) IMT 2000 concept. Due to implementation technology constraints, it has proven difficult to prevent or reduce spurious emission from a mobile transmitter power amplifier, the levels of which can rise as high as xe2x88x9228 dB in the adjacent frequency channel relative to the level in the nominal band. i(his would tend to happen under conditions of worst case manufacturing tolerance and low battery voltage. There are also constraints on the frequency filtering of receivers which mean that the in-band transmission of an ideal mobile can cause interference in adjacent channels.
Statistically these problems are normally unlikely in a random distribution of users in an area served by two (or more) operators, but become significant when a second operator""s mobile happens to come physically close to a first operators base station receiver. Reception of spurious emissions due to a second operator""s mobile at a base station will appear as additional noise and will reduce the base station""s sensitivity resulting either in a reduction of capacity, an increase in the transmission power requirement of its mobiles, and a consequential increase of interference to other nearby cells. The problem, when it does happen, is significant because a large number of users have their communication degraded by a single interfering mobile.
It is believed that the indoor applications of 3G base stations and cells are at particular risk since the physical density of mobile users in, for example, rail stations, shopping malls, is high. Nevertheless, outdoor applications are not immune to such problems.
The areas where it is applicable is in the reduction of interference to the receiver of a base station belonging to a first operator arising from uncontrollable mobile stations subscribing to a second operator which can operate at high level powers, be physically close to, and use an adjacent frequency channel for their transmission. These effects are most likely to arise in time division duplex modes (TDD) of the cellular system but can also arise as a mixed frequency division duplex FDD/TDD situation. Also interference from a second operator""s base station can arise at the first operator""s base station but this issue is limited to TDD mode.
The invention also seeks to provide a system and method for reducing the effects of adjacent channel interference (ACI) in third generation code-division multiple access (CDMA) base station.
The invention also seeks to provide a system and method for reducing the effects of co-channel interference especially in unsynchronized networks and of adjacent channel interference from other operator""s base stations and terminals in TDD mode and from terminals in FDD mode.
In accordance with a first aspect of the invention there is provided a sub-macro base station arrangement for a wireless communication system, the base station comprising a distributed antenna arrangement comprising at least two antennas, wherein the spacing between at least one pair of antennas is at least twice the minimum allowable range for a source of interference to one antenna.
This minimises the risk that more than one antenna is close to a source of interference. This has the advantage that the effective interference from sources of interference can be reduced. The source of interference could be a mobile terminal or a base station. For a frequency of 1900 MHz, a distance of around 15 m will provide sufficient separation for seducing the effects of adjacent channel interference.
The system is suitable for micro-cells, pico-cells and local area networks.
In accordance with a further aspect of the invention at least one of the antennas of the distributed antennas comprises two or more antenna elements having variable phase feeds whereby said antenna is operable to form nulls in its radiation pattern.
In accordance with a further aspect of the invention there is provided a method of communicating in a sub-macro wireless communication cell, wherein the sub-macro base station comprises a distributed antenna arrangement comprising at least two antennas, wherein there is a spacing between at least one pair of antennas is at least twice the minimum allowable range for a source of interference for one antenna, the method comprising, the step of providing simultaneous links between the distributed antennas to a terminal whereby to ameliorate the effect of a source of interference.